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Comparison of the performance of horizontal and vertical flow constructed

wetland planted with Rhynchospora corymbosa

Article  in  International Journal of Phytoremediation · January 2019

DOI: 10.1080/15226514.2018.1488809

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International Journal of Phytoremediation

ISSN: 1522-6514 (Print) 1549-7879 (Online) Journal homepage: http://www.tandfonline.com/loi/bijp20

Comparison of the performance of horizontal andvertical flow constructed wetland planted withRhynchospora corymbosa

O. D. Raphael, S. I. A. Ojo, K. Ogedengbe, C. Eghobamien & A. O. Morakinyo

To cite this article: O. D. Raphael, S. I. A. Ojo, K. Ogedengbe, C. Eghobamien & A. O. Morakinyo(2019): Comparison of the performance of horizontal and vertical flow constructed wetland plantedwith Rhynchospora�corymbosa, International Journal of Phytoremediation

To link to this article: https://doi.org/10.1080/15226514.2018.1488809

Published online: 18 Jan 2019.

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RESEARCH-ARTICLE

Comparison of the performance of horizontal and vertical flow constructedwetland planted with Rhynchospora corymbosa

O. D. Raphaela, S. I. A. Ojob, K. Ogedengbec, C. Eghobamienb, and A. O. Morakinyob

aDepartment of Agricultural and Biosystems Engineering, Landmark University, Omu-Aran, Nigeria; bDepartment of Civil Engineering,Landmark University, Omu-Aran, Nigeria; cDepartment of Agricultural and Environmental Engineering, University of Ibadan, Ibadan, Nigeria

ABSTRACTTreatment performance of horizontal flow (HF) and vertical flow (VF) constructed wetland plantedwith Rhynchospora corymbosa were compared. The average porosity of the CW beds were 0.55,hydraulic retention time (HRT) of 3 days, hydraulic loading rate (HLR) and Organic Loading ratewere 0.058m/day and 3.96 (g�BOD/m2�day), respectively with a volumetric flow rate of 0.14 m3/day. The pollutant concentration of graywater before and after its introduction to the CWs wasmeasured using standard sampling and analyses methods. The mean removal efficiencies (RE)for HF and VF CWs were BOD, 35% and 35.4%; COD, 61.9% and 56.7%; TN, 87% and 92%;TP, 95% and 65%; TSS, 86% and 59.6%; pH, 8.8% and 12.8%, respectively. The graywater washighly contaminated in terms of nutrient and organic load. The mean values of the parameterstested for different CWs were significantly different (P� 0.05). This comparative study favored HFover VF Constructed wetland with HF found to be a viable alternative for graywater treatment fororganics, nutrients and suspended solids removal. The result provided insight into the perform-ance of CWs planted with R. corymbosa.

KEYWORDSConstructed wetlands;graywater; phytoremedia-tion; pollutant removalefficiency; Rhynchosporacorymbosa

Introduction

There has been an unprecedented increase in water con-sumption across the globe. Consequently, studies on waterreusability are gaining interest. Wastewater can be definedas spent or used water with an adversely affected quality(Shah et al. 2014). It has either dissolved or suspended con-taminants in it. Reuse of treated, high-quality reclaimedwastewater for agriculture not only protects human healthbut also serves as a good conservation strategy by reducingthe consumption of limited drinking water for irrigationand reducing fertilizer costs to the agricultural sector inlow-income countries (Zurita and White 2014). Urbanwastewater is usually a combination of domestic effluentwhich consists of blackwater (excreta, urine, and fecalsludge, i.e. toilet wastewater) and graywater (kitchen, laun-dry, and bathing wastewater).

Graywater is biologically polluted effluent and possesses asanitary risk related to a potential spread of microorganisms(Matos et al. 2014). Constructed wetlands (CWs) also knownas treatment wetlands are engineered systems that are devel-oped to improve water quality with relatively low externalenergy requirements and easy operation and maintenance(Wu et al. 2015). These constructed wetlands combinechemical, biological, and physical treatment mechanisms ineliminating heavy metals, pathogens, organic matters,nutrients, and other pollutants that may be present in

wastewater samples (Cui et al. 2010; Babatunde et al. 2011;Zhang et al. 2012). The three basic types of constructed wet-land treatment systems are free water surface (FWS), hori-zontal subsurface flow (HF), and vertical subsurface flow(VF) wetlands. Constructed wetlands have been proven tobe an effective low-cost treatment system for graywater andother wastewater types (Paulo et al. 2009; Hoffmann et al.2011; Wurochekke et al. 2014). Constructed wetlands havebeen widely applied successfully in the treatment of munici-pal wastewater (Mburu et al. 2013), stormwater (�Avila et al.2013), industrial wastewater (Wu et al. 2015), and agricul-tural runoff (Yang et al. 2008). Through constructed wetlandtreatment, organic substances in graywater have greatly beenreduced. Performance of CWs is usually based on theremoval of organics, nutrients, pathogens, and emergingcontaminants (Ramprasad and Philip 2016). Macrophytesare aquatic plants that grow in or near water. They can beemergent, submerged, or floating, and helophytes. They cre-ate conditions for the sedimentation of suspended solids(SS) (Srivastava et al. 2008) in association with the aquaticmicro-organisms and periphytons enhance the uptake ofnutrients from the water (Shelef et al. 2013). Macrophyteshave been reported to transport approximately 90% of theoxygen available in the rhizosphere, which is used by micro-organisms there (Vymazal 2011), utilizes nutrients in waste-water (Zhang et al. 2012) and salt phytoremediation (Shelef

CONTACT O. D. Raphael raphael.davids@lmu.edu.ng, rafdaolu2@gmail.com Department of Agricultural and Biosystems Engineering, Landmark University,PMB 1001, Omu-Aran Kwara State, Omu-Aran, Nigeria.Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/bijp.� 2019 Taylor & Francis Group, LLC

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et al. 2012). This fosters the aerobic decomposition oforganic matter and promotes the growth of nitrifying bac-teria (Shelef et al. 2013). Phragmites australis (Commonreed) is by far the most frequently used plant across theglobe. Several aquatic plants such as P. mauritianus andTypha latifolia, water lilies (Nymphaea spontanea), Azollapapyrus (Cyperus papyrus), T. angustifola, Limnocharis flava,water hyacinth (Eichhornia crassipes), and water lettuce(Pistia stratiotes) have been used as wetlands plants to treatindustrial wastewater with good results (Akinbile and Yusoff2012). Canna indica Linn., C. alternifolius, Flabelliformis(Rottb.) Kukenth, Pennisetum purpureum, Vetiveria ziza-nioides, Acorus calamus Linn., Hymenocallis littoralis Jack.,P. communis, and T. angustifolia Linn. Can. indica, C. flabel-liformis, P. purpureum, and V. zizanioides are fibril root spe-cies (Canna indica possesses rhizomes, but its rhizomes haveno aerenchyma and play a storage role and most of its rootsare fine roots of diameter 1mm) (Chenet al. 2007). In many countries, especially in the tropics andsubtropics, local plants including ornamental species arecommonly used for CWs (Vymazal 2011). Although fernslike Salvinia and Azolla and large algae like Cladophora arewidespread in wetlands (Akinbile et al. 2015), it is usuallythe flowering plants that dominate. Only a limited numberof these plant species are adaptable in CWs in reality.Ewemoje and Sangodoyin (2011) evaluated the effectivenessof three macrophytes (Canna indica, P. australis andSacciolepis africana), under varied hydraulic retention timeand hydraulic loading rate in the treatment of cam-pus sewage.

R. corymbosa (L.) Britton is a tropical perennial plantcommonly called Golden Beak Sedge. Rhynchospora (beak-rush or beak-sedge) is a genus of about 400 species of sedgeswith a cosmopolitan distribution (Strong 2006). A commonweed of swamps often found growing in rice paddies, irriga-tion canals and at the edge of streams in the forest zonesand southern savanna of Nigeria. It has a short root system,short rhizome without stolons and measures about60–150 cm tall (Akobundu and Agyakwa 1998; Taha et al.2015). It is frequently found in sunny habitats with wet,acidic soils. In marshes and savannas, R. corymbosa (L.) maybe the dominant form of vegetation wherever it is found.

Recent studies enumerated 55 individuals in 900 m2 at threelocalities of mangrove communities in South-WesternNigeria (Adekanmbi et al. 2009). The inflorescences (spike-let) are sometimes subtended by bracts which can be leaf-like or showy (Mani 2011). This study is to compare theperformance of HF and VF CW systems planted with R.corymbosa in graywater treatment in the study area. Theneed for the study arises from the increased graywater gen-eration in the study area and the need to select appropriateindigenous wetland plant for its treatment. The soil in thestudy area has poor infiltration property which led to thedeployment of huge resources from time to time to thedraining and evacuation of the septic tank facilities and dis-posal of wastewater into the nearby valley in the study area.Hence, there is a need for the investigation into the possibil-ity of diversion of graywater for possible re-use and recy-cling. There is no known work on the phytoremediationpotential of R. corymbosa as a constructed wetland plant.

Materials and methods

The study area

This study was carried out at the Teaching and ResearchFarm of Landmark University, Omu-Aran, Nigeria. It lies inthe humid plain agro-ecological region of Southern GuineaSavannah of Nigeria. Omu-Aran is on latitude 8�800000 Nand longitude 5�600000 E with an altitude of 564m above sealevel. The climate is a tropical maritime with a long rainyseason and characterized by moderate weather with anannual daily average temperature range from 16–32 �C andaverage annual rainfall of 1000–1150mm spread over6–8months. Graywater used in the study was collected fromthe surface drain in the male and female hostel.

Pilot-scale unit description

Four different units of pilot scale CWs (two for each set)representing HFCW and VFCW were constructed. ThePilot-scale CWs were constructed from 200 kg polyethene oildrums. Each tank has a diameter of 600mm and height of850mm with total volume of 210 L. The drums and otherfittings used were purchased from the local market. Twopilot-scale CWs were formed from a single PE drum bysplitting it longitudinally into 2 halves. The capacity of each

Figure 1. Picture of CW setup for VFCW and HFCW with the drain tap.

2 O. D. RAPHAEL ET AL.

plastic is about 105 L. The configuration produced two par-allel systems, whose treatment capacity can be compared.The setup is shown in Figure 1. The distribution system thatdistinguished VFCW from other CWs was made from a12mm diameter PVC pipe with slots placed on the top ofthe pipes spaced at 80mm, to obtain an even distribution ofwater over the entire bed surface. This was connected to theholding tank by means of flexible hoses. The effluent in theVFCW was drained and recirculated twice in a day for the3 days HRT period. All CW units were operated in batcheswith a capacity of 40 L per time. The selected aquatic plantsR. corymbosa were obtained, trimmed and transplanted intothe CWs in a zigzag planting pattern to regulate the flowpattern. The plant density of eight plants per square meterin each unit was observed. The plants were allowed to accli-matize in the CWs for 8 weeks before investigation com-menced. The wetlands were fitted with an outlet pipe andball valves to control the release of treated water from thewetland. The raw graywater from the hall of residence wascollected and analyzed for various parameters before intro-ducing it into the CWs. The graywater was allowed toremain within the CWs for 3 days (HRT) after which thetreated water samples were collected and analyzed in thelaboratory for the same sets of parameters earlier tested for.

Pre-treatment systemPretreatment system adopted in the primary treatment ofthe raw graywater was slow sand filter sand (filter mediasize was 1–2mm) with a removable cloth bag in a holdingtank placed above the sand bed in the setup. A simple clothbag was placed above the sand filter to remove the floatingand suspended matter and some amount of BOD. Aftereach pre-treatment activity, the cloth bag was removed andcleaned before another batch of graywater was introducedinto the holding tank for further treatment in the con-structed wetland.

Graywater sampling and analysis

A composite sample comprising of the mixture of samplesfrom the two halls of residence were obtained and grab sam-ples collected in sterile bottles for analysis in theEnvironmental Laboratory in Landmark University.Graywater samples for nutrients analysis were collected in2 L polyethene bottles rinsed with distilled water beforehand.All containers were filled to their maximum capacity so thatno air was left inside and were placed in a chest box withice to minimize any change in the parameters. Fecal coli-forms, BOD5 and TSS were determined within 24 hours inthe laboratory. The remaining samples were filtered andkept in the refrigerator at a temperature of 4 �C. All remain-ing parameters were determined within 48 hours after sam-pling except for the BOD5.

Water quality monitoringWater samples were collected from the effluent outlet of allCWs after the 3 day HRT period and analyzed in the

laboratory. Parameter analyzed in the study were pH,Chemical Oxygen Demand(COD), Biochemical OxygenDemand (BOD5), Total Nitrogen (TN), Total Potassium(TK), Total Phosphorus (TP), Oil and Grease (O&G), TotalCalcium (TC), Total suspended solids (TSS), and Heavymetals (Zn, Al, Mg, and Fe). All analyses were carried outaccording to the Standard Methods for the Examination ofWater and Wastewater (APHA 2005).

The substrate

The bed depth of the substrate was 0.2m. It was filled asfollows (from bottom to top):

� The first layer of 50mm consisted of granite aggregates(size was 12–15mm) and was made to cover the inserteddrain pipe fitted with a valve.

� The second layer above the first was of 150mm coarsesand (filter media size was 1–2mm) into which the mac-rophytes was planted.

� Water surface of the average depth of 50mm wasallowed above the sand bed in case the system is to beadapted for a Free Water Surface CW.

� A freeboard of 50mm was allowed to prevent the spillingof water from the CW tanks.

The sand media has a porosity of 55% and Darcy’s con-ductivity of 0.34m/d. The flow rate was adjusted by Bucketmethod with the use of timer and setting of the tap. Theyoung and new plants were acclimatized in the bed mediaand graywater sample was later passed through the bed. Theplastic lined CWs were kept outside in the open spacedevoid of the shade of any form.

Porosity

The porosity of the media (soil) was experimentally deter-mined based on the following relationship which wasreported by Akratos and Tsihrintzis (2007)

P %ð Þ ¼ pd–bdð Þ � 100 (1)where bd¼ bulk density of the soil calculated as the ratio ofdry weight of the soil sample to its volume. pd¼ particledensity calculated as the ratio of the dry weight gravel sam-ple to the difference of volume of gravel and the volume ofwaste required to replace the pores.

Hydraulic retention timeThe hydraulic retention time (HRT) in the wetland was cal-culated with Equation (2) (Crites et al. 2006):

t ¼ LWyn=Q (2)where t is the wetland HRT (day); L is the length of wetlandcell (m); W is the width of wetland cell (m); y is the depthof water in the wetland cell (m); n is the porosity or spaceavailable for water to flow through the wetland and Q is theaverage flow through the wetland (m3/d).

INTERNATIONAL JOURNAL OF PHYTOREMEDIATION 3

Aspect ratioThe aspect ratio selected was based on the pilot scale CWsize of length 800mm to the width of 300mm which is 2.6:1 (i.e. length–width ratio) is derived from Darcy’s Law(Vymazal et al. 1998)

Mass Loading Rate. The following equation was used toestimate the MLR for constituents of interest for each wet-land units: (Alley et al. 2013)

MLR ¼ C � Q (3)where MLR is the mass loading rate, mg/day, C is the con-centration of the constituent of interest in the inflow water,mg/m3, and Q is the flow rate, m3/day.

Pollutant removal efficiency calculationsThe effect of the operational condition on wetland perform-ance was evaluated on the basis of percent removal andmass removal rate. The percent removal (removal efficiency)was calculated as follows (Abdelhakeem et al. 2016)

Removal efficiency %ð Þ ¼ Cin� CoutCin

� 100 (4)

where Cin and Cout is the inflow concentration and outflowconcentration, respectively (mg/L). The mass removal rate(r, in gm2/d) was calculated as follows: (Abdelhakeem et al.2016)

r ¼ q Cin–Coutð Þ (5)where r¼mass removal rate (gm2/d), q¼ hydraulic loadingrate (md�1)

Determination of organic loading rate and hydraulic load-ing rateThe organic loading rate, Lorg (g�BOD/m2�day), was deter-mined using the Equation (6).

Lorg ¼ Cdwgt (6)

Again, C is the BOD (mg/L¼ g/m3) of the influent water,dw (m) is the depth of the medium, which is between0.02–0.5m, t is the detention time. g is the porosity of themedium. This number will indicate the mass of BOD perarea per day that the system is expected to receive.

Hydraulic loading rate was calculated from Equation (7)

HLR ¼ Q m3 d�1ð Þ

SA m2ð Þ (7)

where Q¼ volumetric flow rate, m3/day; SA¼ aerial surfacearea, m2.

Data analysis

Microsoft Excel was used for all descriptive statistical analy-ses. The statistical tests were done using SPSS 22.0 softwarepackage. A significance level of p¼ 0.05 was used for allstatistical tests, and values reported are the mean (avera-ge) ± standard error of the mean. When a significant differ-ence was observed between treatments in the ANOVAprocedure, it was followed by post hoc test. Multiple com-parisons were made using the Least Significant Difference(LSD) test for differences between means. The null hypoth-esis (Ho: lHF¼ lVF) and alternate hypothesis (Ha:lHF 6¼ lVF) at a¼ 0.05 was also tested. Differences betweenthe treatment efficiencies of HFCWs and VFCWs were alsochecked for different pollutants.

Results and discussion

Design parameters calculation results

The average porosity of the CW bed was 0.55 and calculatedHRT, HLR and organic loading rate of HF was 3 days,0.058m/day and 3.96 (g�BOD/m2�day), respectively. Allbased on the highest value of pollutant recorded and volu-metric flow rate of 0.014 m3/day.

Graywater quality and characterization

The average characteristics of influent graywater fed to thewetlands are shown in Table 1. The highest value of72 ± 6.3mg/L, 1120 ± 207mg/L, 21 ± 3mg/L, 101.2 ± 34.6mg/L and 20–101 ± 34.6mg/L were the pre-treatment analyzedvalues obtained for BOD, COD, TN, TK, and TP, respect-ively. Washing detergents are the primary source of phos-phates found in graywater in countries that have not yetbanned phosphorus-containing detergents (Braga and

Table 1. Characteristics and composition of graywater in the study (n¼ 7).a

Parameters Units

Samples

1 2 3 4 5 6 7 Least Highest Mean SE

BOD mg/L 72 68 66 60 57 62 54 54 72 63 6.3COD mg/L 1120 960 704 672 608 613 576 576 1120 750 207.5TN mg/L 21 13.8 14.5 12.9 15.2 20.5 17.2 12.9 21 16 3.2TK mg/L 4.9 3.2 3.1 3 3.2 8.2 4.1 3 8.2 4 1.9TP mg/L 20 32.2 89.2 76.2 96.2 101.2 T. H 20 101.2 69 34.6O&G mg/L 1207 1156 1178 240.2 238 243 220.2 220.2 1207 640 505.4pH – 7.27 6.84 8.32 8.03 8.03 13.05 7.79 6.84 13.05 8 2.1TSS mg/L 40 35 720 500 590 595 550 35 720 433 278.2Zn mg/L 25 9 9.6 7.4 7.4 12.4 12 7.4 25 12 6.1Al mg/L 0.32 0.17 0.19 0.14 0.14 5.04 0.23 0.14 5.04 1 1.8Mg mg/L 28 22 25 18 18 23 29 18 29 23 4.4Iron mg/L 6.6 2.55 2.65 2.05 2.05 7.05 3 2.05 7.05 4 2.2aAverage of 6months sampling operation, SE: standard error.

4 O. D. RAPHAEL ET AL.

Varesche 2014). Hence the elevated value of TPwas measured.

The values obtained for O&G was in the range220.2–1207 ± 505.4mg/L. The value was high even thoughstudents were not allowed to cook in the hall of residence,the unexpected high value was due to the fact that most ofthe body care products are oil based and wastewater fromthe washing of cutleries contains a high percentage of oiland these are washed down the drain. The highest value ofTSS obtained was 720mg/L. For the heavy metals of Al, Mg,Zn, and Fe their values were 0.14–5.04 ± 1.8, 18–29 ± 4.4,7.4–25 ± 6.1 and 2.05–7.07 ± 2.2mg/L, respectively. Theheavy metal content of the graywater was relatively lowespecially the Al. One reason for the elevated values of someof the metals in the graywater could be some chlorine tab-lets that had been used for disinfection of water in themetallic storage tanks and plumbing fittings. These disinfect-ant tablets are acidic and that may cause leaching of zincfrom the plumbing (Eriksson et al. 2002; Eriksson andDonner 2009). The pH ranged from 6.84 – 13.05 ± 2.1 in analkaline direction. The characteristic of graywater was foundsimilar to the results in different parts of Africa(Wurochekke, et al. 2014; Katukiza et al. 2015).

Organic matter removal (COD, BOD, TSS) in HFCWand VFCW

The outlet concentration of COD and TSS varied signifi-cantly in terms of pollutant removal efficiency (RE) whichwas as high as 61.9% and 86%, respectively in HFCW. Themean value of COD RE of 46% was recorded while the aver-age value of TSS was 71%. As for the BOD, the highestvalue of 35% was recorded for the RE. The low BOD REcan be attributed to the low biodegradability of the gray-water from the study site. The results of the analysis of thepollutant removal (PR) for samples treated in the HFCWare as shown in Table 2. The result was different from the

general view that HF supports high organic removal as inTuszy�nska and Obarska-Pempkowiak (2008) in whichremoval of organic matter and suspended solids in a HFCWtreating domestic sewage varies from 72% to 95% for SS,71.2% to 94.1% for BOD5 and from 59.7% to 89% for COD.For this system, low BOD RE can be related to the root sys-tem of the plant R. corymbosa. The root system is in general,perennials with short, hardened, knotty, sometimes horizon-tally creeping rhizomes without stolons (Taha et al. 2015). Amassive network of roots and rhizomes maintain a high bio-logical activity in the constructed wetland, due to their abil-ity to transport oxygen from the leaves to the roots isessential (Hoffmann et al. 2011). The wide rooting zone andvast biofilm surface area of P. australis as reported in Baskaret al. (2014), resulted in its higher BOD removal. For HFCWs a uniform distribution of roots in the entire filter bedis important, Vymazal (2011) reported that root-derived aer-obic dynamics is very limited in HFCWs and its role isminor in periodically loaded VFCWs with short hydraulic-retention times. Since microorganisms are considered keydrivers in the treatment process, any factor that changestheir composition, biodegradation efficiency or concentra-tions has a significant impact on the whole CW treatmentefficiency (Shelef et al. 2013).

The performance of the VFCW in treating graywaterinfluent (summary) is shown in Table 3. The mean RE forBOD, COD, and TSS in the VFCW were found to be 35.4%,56.7%, and 59.5%, respectively. These values were better orhigher than what was observed for the Organic matter indi-cators in the HFCW but still relatively lower compared towhat has been reported in Wurochekke et al. (2014) butagrees with the Paulo et al. (2009). The low BOD RE agreeswith what was observed in Haghshenas-Adarmanabadiet al. (2016).

The average COD removal in Paulo et al. (2009) wasabout 48% while removal of turbidity and TSS were 81%and 84%, respectively. The retention of influent in the bed

Table 2. Percentage pollutant removal efficiency for all samples in HFCW.

Parameters UnitsSamples (% RE)

Least (% RE) Highest (% RE) Mean SE1 2 3 4 5 6

BOD mg/L 16.7 16.7 31.8 35 26.3 22.6 16.7 35 25 7.3COD mg/L 28.6 28.6 59.1 61.9 52.6 47.8 28.6 61.9 46 13.2TN mg/L 86.7 86.7 78.6 74.4 77 81 74.4 86.7 81 4.7TK mg/L 57.1 57.1 35.5 53.3 34.4 75.6 34.4 75.6 52 17.1TP mg/L 3 3 78.8 83.7 78.3 95.3 3 95.3 57 33.6O&G mg/L 26.8 26.8 46.6 11.1 7.1 12 7.1 46.6 22 16.3pH 3.8 3.8 2.6 12.7 10.5 4.5 2.6 12.7 6.3 17.3TSS mg/L 47.5 47.5 84.7 86 84.7 74.8 47.5 86 71 16.3Zn mg/L 88 88 85.4 91.9 70.3 87.1 70.3 91.9 85 8.3Al mg/L 65.6 65.6 94.7 0 100 99.6 0 100 71 42.7Mg mg/L 64.3 64.3 92 0 94.4 95.7 0 95.7 68 40.8Iron mg/L 83.3 83.3 73.6 82.9 61 88.7 61 88.7 79 10.9

RE: removal efficiency; SE: standard error.

Table 3. Mean removal efficiencies (%) for all CWs summary.

CW systems pHParameters (mg/L)

TSS BOD COD TN TP O&G TC ZN AL Mg Fe

HF effluent (%) 8.8 86 35 62 87 95 47 0 92 100 96 89VF effluent (%) 12.8 59.5 35.4 56.7 92 65 22.6 0 91 32.3 82.5 93.8

INTERNATIONAL JOURNAL OF PHYTOREMEDIATION 5

for biological treatment was interfered with due to the inter-mittent reintroduction of influent to the system therebyleading to poor BOD removal and TSS.

Nutrient removal (N and P) in HFCW and VFCW

The results from the HFCW experiment show that thenutrient constituent of the graywater was highly removed.The average TN, TK, and TP were 81%, 52%, and 57%.However, the percent P.R of Nitrogen was exceptionallyhigh. This is as a result of the shallow depth of the substratein the pilot scale HFCW or other reasons which enhancethe presence of oxygen in the system i.e. aeration. Plantgrowth leads to removal of nutrients such as nitrogen andphosphorus, as the oxygen transport into HFCWs is limited,enhanced nitrification cannot be expected. However, denitri-fication can be very efficient, (Hoffmann et al. 2011).Phosphorus removal can be achieved in CWs by adsorptionand precipitation, and a small amount is also taken up byplant growth. It has been proven that HFCW can effectivelyremove 50–60% of nitrogen removal due to the limited oxy-gen transfer inside the wetland bed (Kantawanichkul andWannasri 2013). The RE for TN and TP in the studiedVFCW was 92% to 65%, respectively. Plant growth leads toremoval of nutrients such as nitrogen and phosphorus(Hoffmann et al. 2011). In VFCWs with sufficient oxygensupply, ammonia can be oxidized by autotrophic bacteria tonitrate; this process is called nitrification. An almost com-plete nitrification with 90% ammonia oxidation is com-monly reported for VFCWs. It the studied systems theretention period of 3 days and twice-a-day reintroductionmust have contributed to high TN removal. Studies indi-cated that nutrient removal was better at a higher tempera-ture (Prochaska et al. 2007). Phosphorus removal can beachieved in CWs by adsorption and precipitation, whichmost times depends on the type of substrate used only smallamount is also taken up by plant growth (Hoffmann et al.2011) hence the low RE for TP.

Heavy metal removal in HFCW and VFCW

The average RE for the heavy metals in the graywater wasfound to be high for Zn, Al, Mg, and Fe with the values of81%, 71%, 68%, and 79%, respectively. Heavy metal loadsfrom bathroom graywater are small in comparison with typ-ical municipal wastewater loads but still do not always meetenvironmental quality standards for surface waters (Erikssonand Donner 2009). Heavy metals removal has been reportedat 42% for manganese, 75–99% for cadmium, 26% for lead,75.9% for silver and 66.7% for zinc. These are removed byadsorption and absorption into the filtration matrix and theleaves, shoots and rhizomes of the wetland plants (Odingaet al. 2013). This indicates that R. corymbosa (Cyperaceae)plants with rhizomatic roots thicker and fewer roots has anaffinity for heavy metal absorption. The treatment systemrecorded a high RE for all heavy metals monitored. For Zn– 91%, Al – 32.3%, Mg – 82.5%, and Fe – 93.8%. Al is theheavy metal with least concentration in all the influent

sampled. There is a number of wetland processes thatremove heavy metals; namely, binding to soils, sedimenta-tion and particulate matter, precipitation as insoluble salts,and uptake by bacteria, algae, and plants (Kadlec andKnight 1996). The high RE for all heavy metals confirms thefact that R. corymbosa (Cyperaceae) plants roots have anaffinity for heavy metal absorption.

Comparison of the removal efficiencies of the CWs

The statistical analysis shows HFCW did well in the removalof TP, O&G, pH, Al and Mg. VFCW was good for TN andFe removal only but always ranking next to HF in perform-ance. If all the heavy metals monitored in the study are con-sidered to be no issue because of their detectable amount ingraywater, HF will be better selected in handling other pol-lutants. The ANOVA analysis shows that the null hypothesisthat the means of RE for the two (2) systems are equal (Ho:lHF¼ lVF) should be rejected and alternate hypothesis isaccepted (Ha: lHF 6¼ lVF) at a¼ 0.05. That is the two meanvalues were significantly different. The decision on which ofthe two systems to select for graywater treatment will bebased on inflow concentration, operation and maintenance,cost of construction and ancillary works, treatment goalsand environmental impact of the systems. Generally, gray-water does not usually contain an elevated concentration ofrecalcitrant pollutants as most pollutants are easily treatable.

Comparative studies have always favored HF over VF(Konnerup et al. 2011; Zhang et al. 2014). HF perform bet-ter in environmental impact (odor, Mosquito control andaesthetically appealing) low operation and maintenance cost(most times passive), low cost due to non-requirement ofancillary works like distribution pipes and intermittentreloading system (pump) and sometimes because of the agri-cultural reuse purpose of the effluent inefficient nutrientremoval is always a non-issue. Figure 2 shows the graphicalcomparison of removal efficiencies of CWs. It can bededuced that for most parameters compared HFCW has bet-ter RE compared with VFCW.

Conclusion

Constructed wetlands with HF or VF are a viable alternativefor graywater treatment for organics, nutrients and

0

20

40

60

80

100

120

BOD

COD TN TK TP

O&

G TC pH TSS Zn Al Mg

Iron

Rem

oval

effi

cienc

y, %

Monitored parameters

HF Mean % R.E VF Mean % R.E

Figure 2. Graphical comparison of removal efficiencies of CWs.

6 O. D. RAPHAEL ET AL.

suspended solids removal. The mean removal efficiencies(RE) for HF and VF CWs were BOD, 35% and 35.4%;COD, 61.9% and 56.7%; TN, 87% and 92%; TP, 95% and65%; TSS, 86% and 59.6%; pH, 8.8% and 12.8%, respectively.Heavy metal (RE) was between 89–100% and 32–93.8% forHF and VF CWs, respectively. The odor and vector controloffered by the HF concept make it attractive. The HRT�3 days is good for efficient pollutant removal in all CWstudied. Plants are an important component of wetland sys-tems. R. corymbosa as a macrophyte does not have aerialplant tissues, roots vast enough to provide surface area formicrobial growth and oxygen exchange compare to otherpopularly used emergent macrophytes like P. australis, Can.indica, T. latiforal etc. The treated effluent did not meet theFEPA Nigeria standard (FEPA 1991) but the result providedinsight into the performance of CWs planted with R. corym-bosa. It is necessary to combine different systems in a hybridform to treat graywater efficiently. The results show that(HF–VF) hybrid system will combine the good parts of thetwo CWs into an integrated system.

Acknowledgments

We thank the management of Landmark University for the provisionof adequate laboratory equipment and the commitment of the labora-tory staff and assistants. We gratefully acknowledge the critical com-ments and corrections of respected reviewers whose comments andcorrections improved this work considerably.

Disclosure statement

The authors declared that there is no any conflict of interests.

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http://www.ijehse.com/content/12/1/106http://www.ijehse.com/content/12/1/106https://doi.org/10.1007/s11157-008-9135-xhttps://www.researchgate.net/publication/330479385

AbstractIntroductionMaterials and methodsThe study areaPilot-scale unit descriptionPre-treatment system

Graywater sampling and analysisWater quality monitoring

The substratePorosityHydraulic retention timeAspect ratioPollutant removal efficiency calculationsDetermination of organic loading rate and hydraulic loading rate

Data analysis

Results and discussionDesign parameters calculation resultsGraywater quality and characterizationOrganic matter removal (COD, BOD, TSS) in HFCW and VFCWNutrient removal (N and P) in HFCW and VFCWHeavy metal removal in HFCW and VFCWComparison of the removal efficiencies of the CWs

ConclusionAcknowledgmentsDisclosure statementReferences